Sylvester Stallone's private security team sprung into action yesterday when an overzealous fan allegedly snuck on to Sly's L.A. estate and tried to score an impromptu meet-and-greet with the star.
Law enforcement tells TMZ ... a member of Stallone's private security team called police around 3:30 PM and told cops they had performed a citizen's arrest on a 22-year-old male who got past Sly's gates.
We're told the suspect claimed that he simply wanted some face time with "The Expendables" star -- who wasn't home at the time.
The LAPD eventually arrived at the scene -- where they took the man into custody and booked him for misdemeanor trespassing.
According to police, the man is still in custody and is being held on $2,500 bail.